I teach in a year group that is not Y2 or Y6, so have been teaching the new curriculum for a year.
IMO - and it is only an opinion - it can take a while for schools whose primary means of differentiation for the more able has been 'acceleration into the written / taught curriculum for years ahead' to get into the idea of what 'mastery / breadth' as a means of differentiation looks like. Other schools have always differentiated in a less 'linear' way and have found it easier to adapt.
The written maths curriculum is a single pathway through the world of Maths, and by definition there is lots of stuff that has 'not been chosen for inclusion', although it is at the right level mathematically. One means of extension is to explore some of this 'unincluded' Maths at a similar level.
Another, as sneepy says, is to explore increasingly open-ended application of the same mathematical content. So from adding 2 digit numbers, the next 'linear' step might be adding 3 digit numbers,, but more open-ended application might be first to use addition to find perimeters of shapes, and then to find a range of shapes with exactly the same perimeter, next to generate systematic rules for generating families of such shapes. then you can go back to the 2 digit addition again and go off in a different direction to do with word problems.
With time, new and old resources that to extend Maths in this 'non linear' way are becoming available / remembered / linked to schemes of work - but as i say, for those schools that have ONLY used 'go onto the work taught in the next year' - or 'go into the next classroom' - as extension for the more able, the changeover in mindset and approach takes longer.
Yes, of course, there are also children who need acceleration onto the next year group's material - the child who was doing A-level Maths before leaving Primary would be an example - but there is a LOT that can be done which is genuinely interesting and challenging mathematically before that approach is required.